1. Field of the Invention
The present invention relates to a method for forming a blocked cross-plied polymer film. More particularly, it relates to a tubular extrusion and blocking method which produces a polymer film having a reduced machine direction (MD) orientation and an increased transverse direction (TD) orientation of the polymer molecules in two extruded layers, with the transverse direction orientation of the molecules of one layer of the film crossing the transverse direction orientation of the molecules of another layer of the film to produce a balanced cross-plied film.
2. Discussion of the Prior Art
Generally, when a single-layer or laminated plastic film is formed, for example of a high density polyethylene, it is extruded from a tubular extrusion die having stationary die lips. The extruded film is primarily unbalanced as the molecules of the polymer melt are principally oriented in a longitudinal direction of the film, commonly called the machine direction (MD). The molecular orientation in the transverse direction of the film is low. As a result, the resulting film has low strength to stresses applied in a direction deviating from the machine direction of the film.
In order to improve the physical strength and characteristics of an extruded film, attempts have been made to reduce the unbalanced film orientation by achieving a greater transverse direction molecular orientation and a reduced machine direction molecular orientation in the extruded film so as to achieve a better balance of the two.
Generally, transverse direction orientation of the molecules can be produced to some degree by blowing the film after extrusion thereby stretching it in a transverse direction. However, the limits to which a film can be expanded, the socalled blow up ratio (BUR), without breaking or becoming too thin severely limits the amount of transverse orientation which can be imparted during the expansion process.
Attempts have been made to achieve a balanced cross plied structure in the wall of an extruded film by rotating an entire die during extrusion of a film. This technique is disclosed in U.S. Pat. No. 4,358,330. While this technique has some merits in producing a transverse molecular orientation which can be further enchanced during expansion of the film, the die and supporting apparatus which are required are complex and expensive to build and maintain. Moreover, since the molecular orientation imparted by die rotation takes place outside the die, this too limits the amount of transverse direction orientation which can be obtained, as any such orientation must be done before the film frost line is reached. Furthermore, excessive twisting of the thermoplastic melt will cause the tubular film to collapse, making it difficult to implement this type of orientation technique.
It has also been attempted to bond separate film layers together each having different molecular orientation patterns in order to produce a resultant film of desired structural characteristics. This method does not use in-line techniques since the film layers are separately produced and processed and then bonded together. Since the process is not in-line, it requires additional processing and handling of the film layers which is costly and undesirable.